Ore Geology Reviews ( IF 3.2 ) Pub Date : 2023-08-10 , DOI: 10.1016/j.oregeorev.2023.105601 Shuo Wang , MingJian Cao , GuangMing Li , Noreen J. Evans , Wangdui Silang , KeZhang Qin
The Zhibula skarn deposit is a Cu polymetallic deposit, located in Eastern Gangdese, Tibet. In recent years, research on this deposit has mainly focused on lithology, geochemistry, ore-forming fluid and mineralization age, whereas there has been a relative lack of focus on the occurrence of Co, Te, Au and Ag. Three paragenetic stages were distinguished based on mineralogy and textural relationships: Stage I prograde skarn with formation of garnet, pyroxene, magnetite and wollastonite; Stage II early retrograde skarn comprising garnet, tremolite and epidote; late retrograde Stage III characterized by various sulfides with chlorite, quartz and calcite. Photomicrographs and backscattered electron images of thin sections confirm that Co, Te, Au and Ag mineralization are usually associated with chalcopyrite and bornite, which developed during the late retrograde skarn stage (III). The independent Co mineral siegenite was identified along with isomorphous substitution of Co in sulfides and oxides. Compared with chalcopyrite (mean Co content = 10.8 ± 5.32 ppm, 1σ, n = 43), both pyrite and magnetite typically contain obviously higher Co contents (2454 ± 1837 ppm (1σ, n = 14) and 228 ± 49 ppm (1σ, n = 19), respectively). The correlations between Co and other elements indicate that Co replaced Fe in pyrite via isomorphous substitution. The mineral assemblage of independent Co, Te, Au and Ag minerals (siegenite, hessite, calaverite, petzite) with bornite and chalcopyrite also indicates coeval Co, Te, Au, Ag and Cu mineralization during the late retrograde stage. Assuming a Cu-Co precipitation temperature of ∼ 300 °C (supported by previously published fluid inclusion microthermometric results), the mineral assemblage of siegenite and hessite with pyrite and galena reveals an ore-forming fluid logfTe2 of −14.9 to −8.3 and logfS2 of −11.4 to −10.5. The Zhibula deposit and the adjacent well-studied Qulong deposit have similar magmatic source characteristics and mantle-derived magma was likely involved in the formation of mineralization at Zhibula. Compared with previously published trace element data from other skarn deposits, chalcopyrite, pyrite and magnetite in the Zhibula skarn deposit have obviously higher Co contents (8 ∼ 20 ppm, 1500 ∼ 5000 ppm and 160 ∼ 300 ppm, respectively). Accordingly, we propose that trace element analysis of these minerals can be used as a general indicator of Co enrichment in skarn deposits.